1. Field of the Invention
This invention relates to laser systems and associated fabrication methods for directing a laser beam away from a substrate, and more particularly to laser systems that can be monolithically integrated with electronic circuitry on the same substrate to optically communicate with adjacent substrates.
2. Description of the Related Art
Ultra-high speed interconnect links between integrated circuit (IC) chips and data buses will be needed for contemplated 3-dimensional opto-electronic systems. Currently available electronic systems incorporate optical isolation and optical data parts between larger subsystems, but the optical elements are discrete. A more compact, less expensive and more reliable system would result if the optical elements could be monolithically integrated on the same chip substrates as the electronic circuitry.
There are three known approaches to achieving out-of-plane laser emission for potential optical interconnect applications. The first is the use of vertical cavity lasers, in which the laser beam is initially emitted vertically upward and away from the substrate upon which the laser is formed. This type of laser is described in Tell et al., "High-power cw vertical-cavity top surface-emitting GaAs quantum well lasers", Applied Physics Letters, Vol. 57, No. 18, 29 Oct. 1990, pages 1855-1857. They are fabricated by reactive ion etching (RIE), using epitaxially grown Bragg reflectors for mirrors. However, despite rapid progress in the development of GaAs/AlGaAs and GaInAs/GaAs vertical cavity surface emitting lasers (SELs) in recent years, such lasers exhibit poor efficiency and are subject to significant diffraction problems. Also, the development of InGaAsP/InP vertical cavity SELs has been hampered by the difficulty of realizing high reflectivity semiconductor quarter-wavelength Bragg mirrors, due to the low index of refraction modulation in such mirrors.
The second technique for out-of-plane laser emission involves the fabrication of a periodic grating on the laser's upper cladding layer to couple light vertically out of the laser plane. For example, see Itaya et al., "New 1.5 Micron Wavelength GaInAsP/InP Distributed Feedback Laser", Electronics Letters, Vol. 18, No. 23, 1982, pages 1006-1007. Unfortunately, Bragg reflectors of this type suffer from inherent inefficiency, since it is difficult to couple a high percentage of the gain out into the vertical direction. In addition, although they satisfy the Bragg condition, extraneous orders of propagation do not deflect their output in the required direction, and therefore a rather broad spatial distribution is typically obtained.
A third approach is to use in-plane SELs, in which a laser beam is initially generated along an axis parallel to the substrate, and then deflect the beam off a turning mirror so that it travels away from the substrate. Such a system is illustrated in FIG. 1. A laser 2 extends upward from a semiconductor substrate 4, with an active lasing region 6 sandwiched between semiconductor cladding layers 8 and 10; the body of the substrate can itself serve as the lower cladding layer. A trench 12 is formed behind the laser to allow a fully reflective mirror 14 to be coated over its rear surface, while an angled trench 16 is formed immediately in front of the laser to permit the deposition of a partially reflective mirror 18 over the front end of the laser. The trench wall 20, which establishes the opposite side of the trench from the laser, is formed at an angle that causes at least part of the emitted laser beam 22 to be deflected generally perpendicular to the substrate. Since the laser beam 22 expands in the vertical direction as seen in FIG. 1 (it also expands laterally into and out of the page), the reflected beam 22a is similarly divergent.
Three different fabrication techniques have been used to form this type of turning mirror. They involve angled flood ion beam etching (see, e.g., Wakabayaski, "In GaAsP/InP horizontal cavity surface-emitting lasers radiating in two opposite directions", Applied Physics Letters, Vol 61, No. 13, 28 Sep. 1992, pages 1499-1501); mass transport (see, e.g., Liau, "Low threshold GaInAsP/InP buried-hetero-structure lasers with a chemically etched and mass-transported mirror" Applied Physics Letters, Vol 44, No 10, 15 May 1984, pages 945-947); and selective laser or electron-beam resist exposure coupled with ion milling (see, e.g , P. D. Maker et al , "Phase Holograms in PMMA", Journal of Vacuum Science and Technology, Vol. B10, November-December 1992, presented at EIPB 92 Symposium, Orlando, Fla., May 1992). Each of these techniques, however, is less than optimum in terms of flexibility, simplicity and precise 3-dimensional beam directional control. With resist masking and angled ion beam etching, for example, curved surfaces in only one dimension have been achieved. In addition, different curvatures and turning angles cannot be achieved on different devices without remasking steps. Multiple resist masking steps and mass transport suffer from process complexity, high fabrication temperatures that are beyond the temperature limits of electronic circuitry that might otherwise be placed on the same substrate, and a poor control over the final optical profile and beam angle. Resist "holography" and ion beam etching also suffer from process complexity and a lack of precise control over the final optical profile, due mainly to proximity effects, laser resolution, resist development characteristics, ion etching nonuniformities through resist masks, and resist-to-substrate etch rate differences.
A focused ion beam (FIB) has also previously been used to form a flat turning mirror. See, e.g., Harriott et al., "Micromachining of optical structures with focused ion beams", Journal of Vacuum Science Technology, Vol. B5, No. 1, January/February 1987, pages 207-210. The ion beam is raster scanned across the substrate immediately adjacent to the output end of the laser, with the number of scans linearally decreasing as the distance from the laser increases, to form the angled wall 20. In practice, this does not result in a precisely flat turning mirror, since a residue 24 of redeposited substrate material is built up towards the bottom of the laser. However, since the mirror 20 is formed immediately adjacent the laser, the emitted beam 22 strikes only the central flat portion of the mirror, and the residue 24 does not significantly affect the beam's reflection.
The use of a turning mirror with an in-line (parallel to substrate) laser has thus far not been practical for use as an inter-substrate optical link in an integrated 3-D, multi-chip opto-electronic system. In such a system it would be highly desirable that an optical detector on one substrate, used to detect an optical transmission from another substrate, be as small as possible. This saves chip area, and perhaps more importantly reduces capacitance effects associated with larger detectors. Such capacitance effects slow the detector's speed of response, and make it unsuitable for high speed or large bandwidth operations. In addition to the fact that the existing turning mirrors deflect the laser beam as a divergent beam, thus increasing the required detector dimension, the beam is emitted from the laser over a rectangular emission area that is much wider than it is high. Thus, in FIG. 1 the laser beam will actually have a considerably greater dimension into the page than its vertical height when initially emitted. This lateral beam dimension also diverges, and greatly adds to the required detector dimension.